Question about 4wheel drive.

[shibby]

I just got my jeep about 3 days ago, so I'm pretty new at all this. It's a 2003 (used) and I wanted to make sure there was nothing seriously wrong with it. Yesterday while in a cul-de-sac that could have doubled for a skating rink I had to put it in 4H to get myself moving (had a slight grade too). When I got off the ice onto the dry pavement I had to do a hard turn. When the wheel was cranked all the way over it started jerking really hard, enough to really startle me. Seemed to stop as I straightend out again. I tested this a few times and it does it both to the right and left in 4H, but not in 2H. When in snow or ice it doesn't seem to do this.

Is this normal, or should I be worried that I bought damaged goods?

[BlackYJ]

There is nothing wrong.  The four wheel drive requires slip on the surface, so while turing on dry pavement in 4x4 it happens.  The reason is the tires are trying turn at a different rate and therefore the outer tire hops trying to catch up.  Actually you can you this as a test to see if you 4x4 is working

[Robb]

When you are turning, your outside wheel has to travel a longer distance than your inside wheel.  This is fine when you are on ice, because the wheels are slipping to make up the difference.  When you are on a hard packed surface, your wheels don't give, and you will either bind up your drive train, or your tires will hop to "adjust themselves".

Avoid driving in 4wd on hard packed surfaces.

[fony3]

The 4wd is designed to run both the front and back at the same speed when engaged. When you do a tight turn, the front and back take different turning archs needing differnet speeds. I believe that this causes your drive train to "wind-up" and build up torque. The hopping is caused by the realease of some of the torque.

If you were on gravel or ice, the tires would be allowed slip more, and not hold the drive train in torque. On dry roads your tires grip alot more, causeing the wind up. I believe that it can be very damaging, and you really shouldnt have 4wd engaged on dry pavement.

This is just info i gathered from some research some time back, and some information from a buddies dad.

[shibby]

ah... well that takes a load off my mind.  I've driven a few 4x4's just never a jeep and they never had that issue. Probably because one was a full time 4X (J20 3/4 ton), the other was a GMC Sierra 2500 and pretty sure it sends different power to the front and rear wheels.

[FiEND]

it should be front and rear tires, diffs would allow inside and outside tires to be OK, but front and rear would bind.

our liberty has selec-trac so can put it in 4 hi full-time to unlock the front to rear in the transfer case so we can drive no probs on dry pavement and do any turns.  4 hi part-time locks the transfer case front to rear.

binding like that is pretty hard on the driveline is it not?  so probably not something you want to do often if possible.

anyone know if the 06 or 07 TJ has a selec-trac option or similar?  its a great feature.

[Bnine]

anyone know if the 06 or 07 TJ has a selec-trac option or similar?  its a great feature.

It is nice to have, but they can be really prone to failure. Pretty common in the grands. I dont know if its the same set up nowadays.

And wranglers are still solid lockup, no fancy stuff.

[FeatherFoot]

The 4wd is designed to run both the front and back at the same speed when engaged. When you do a tight turn, the front and back take different turning archs needing differnet speeds. I believe that this causes your drive train to "wind-up" and build up torque. The hopping is caused by the realease of some of the torque.

Correct answers all, however;

As pointed out above, turning in locked up 4X4 forces tires to rotate at speeds either faster or slower than they would if they were free wheeling.  A tire which is skidding or spinning on ice, looses lateral stability.  Therefore, when driving on ice, you will not notice "crow hop" from torque buildup, but your lateral stability and turning ability is compromised.  If you do some snow/ice cul-de-sac turn tests you will see.  This unexpected problem is often the reason for seeing a disproportionate number of 4X4's in the ditch during icy driving conditions.  The skidding/spinning of the tires is relatively mild, however a curling rock dosen't rotate very fast to achieve a disparity in grip.

It's great for straight line accileration, but beware of turns or unexpected skids on ice.

And yes, the select trac system is great on road driving snow and ice because all the wheels get torque but each is allowed to rotate at its own speed.

[Bnine]

Wow,  Bob.

The reason you see 4x4's in the ditch is excessive speed due to over confidence, and the lack of know how on driving a 4wd.

People habitually let off the throttle, and some even touch the brake when they realize they have entered a corner to fast. As bad as that is in most vehicles, it is twice as bad in a 4wd. Not because your wheels are turning at different speeds, the differentials take care of that. But because there is torque applied to both axles, which causes the vehicle to react differently then a rear, or front wheel drive unit.

If you are in a corner in a 4wd, you need steady torque applied to the drive axles at a minimum, and if it starts to let loose you need to drive into it. Letting off transfers to much torque and weight, and away you spin.

Driving down the road, your vehicle in 4wd does not have an issue with wheels turning at different speeds (unless your diffs are welded). The wheel hop you get at slow speed and full turn is a result of the front traveling significantly farther then the rear while the driveshafts spin at the same rate.

That is not the same thing as driving down a road and rounding a bend. For the most part, your tires still actually travel the same distance front and rear, just the inside and outside travel different distances, again, taken care of by the differential.

The selecttrac type of 4wd minimizes all these effects by limiting the torque dilivered to the front axle, not to all 4 wheels. The power to individual wheels is still controlled by the differentials.

In some fancier systems like chevs, and maybe some jeeps now, that axle torque is combined with a traction control system that acts on the brakes of the individual wheels, essentially controlling which wheels get torque.

I dont know which of the two the newer jeeps use, but I do know the older ones like the part/fulltime xj's do not use the traction control combined with the selectrac case.

Sorry to be so long winded.

[FeatherFoot]

Hey Bill,
 I think we're pretty much on the same page here but I my point on corners is relevant with sharper turns, ie city streets etc., but it is also relevant on highway driving where, due to ruts, crosswinds or whatever, the vehicle gets more than just a little askew.  

In this case, and for ease of calculation, we will say the vehicle gets 45 degrees out of straight line.  The driver will intuitively steer in the direction of travel.  The front wheels (and diff) are rotating at the vehicle speed.  The rears (and diff), at 45 degrees to the line of travel, want to rotate at a speed half that of the moving vehicle (a nutral, optimum traction speed), but are forced to rotate at the same speed as the fronts due to the locked transfer case.  The rears are effectively 'spinning'.  This is not something the differentials can compensate for.

The 'spinning' wheels have no lateral traction and since the rear of the vehicle is swinging out of the direction of travel, the inertia will pull the vehicle into a 360.  Slacking off the throttle will accentuate this motion, (as Bill eluded to) and few people have the experience and guts to accelerate out of it.

A small deviation from straight line has minimal impact, but the further the deviation from straight line, the more pronounced the effect.

So I agree the problem starts with overconfidence, but the results of getting into a slide is probably a new unexpected and uncontrolable scenario for drivers of units which force the front and rear diffs to rotate at the same speed.

[Bnine]

Hey Bill,
 In this case, and for ease of calculation, we will say the vehicle gets 45 degrees out of straight line.  The driver will intuitively steer in the direction of travel.  The front wheels (and diff) are rotating at the vehicle speed.  The rears (and diff), at 45 degrees to the line of travel, want to rotate at a speed half that of the moving vehicle (a nutral, optimum traction speed), but are forced to rotate at the same speed as the fronts due to the locked transfer case.  The rears are effectively 'spinning'.  This is not something the differentials can compensate for.

Again Bob, extremely exagerated. The difference in wheelspeed in highway and typical road corners is minimum. Not at all like cranking the wheels turning around in a circle.

If that where to happen, (half wheel speed) you would feel the binding in normal driving as well as tight tight turns, any time you drove in 4wd on a dry surface. To do that, you would need to spool both differentials. Just doesnt happen.

Simplest analogy.

Rear wheel drive comming to a stop. Gear down at high rpm and let out the clutch and what happens? The rear slides out. It slide out due the sudden resistance dramatically breaking traction.

Do that with power to both axles and yes, like you said, it will exagerate that and inuduce a 360.

Driving on normal roads the only effect you will notice due to power to both axles is a bit of understeer, or front end shoveling ahead. All a result of what you are talking about, but not a common thing to throw people in the ditch.

The 'spinning' wheels have no lateral traction and since the rear of the vehicle is swinging out of the direction of travel, the inertia will pull the vehicle into a 360.  Slacking off the throttle will accentuate this motion, (as Bill eluded to) and few people have the experience and guts to accelerate out of it.

A small deviation from straight line has minimal impact, but the further the deviation from straight line, the more pronounced the effect.

True, but you notice more understeer. Only way people throw themselves into spins with this, is panic and letting off the fuel or braking.

Not that you are technically incorrect. You just arent approaching it from a practical angle.

The science of why a 4x4 handles differently is great, but that doesnt explain to someone how to drive one  :wink:

been fun

[FeatherFoot]

The key word in my message is ICE.  Different than mud or dry.

been fun

That's what this is about.  

Merry Christmas!